This invention relates to a method for producing in high purity a phenolic resin which is superior in thermal resistance, resistance to solvents, acid proofness, moisture proofness and electrical properties and which may be employed satisfactorily as an encapsulating material for semiconductor chips or packages or as a starting material for preparation of printed circuit substrates.
Recently, with a rapid progress in scientific technology, centered about electronic industry, the demand on various products and the starting materials therefor has become more strict. Above all, the progress in the field of the semiconductor-related technology is outstanding such that the degree of integration of semiconductor memory devices has become higher. In keeping therewith, the tendency towards miniaturization of interconnect, ion and a larger chip size, and transition of the packaging method from through-hole packaging to surface packaging is predominant. However, in an automated surface packaging line, problems are presented in that semiconductor packages undergo rapid changes in temperature during soldering of lead wires such that; molded parts of the resin used for encapsulation of semiconductor chips are susceptible to cracking to deteriorate the interface between the lead wire resins to lower the moisture proofness of the semiconductor packages.
With a view to obviating the lowering in moisture proofness of the semiconductor packages, a method has been proposed for increasing the degree of filler packing by changing the filler shape. However, such method suffers from the problem that the elastic modulus of the resin is increased so that cracking tends to be incurred by thermal impact. For reducing the thermal impact on immersion or the semiconductor packages in a soldering bath, a method has been proposed for modifying the resin composition by addition of silicone compounds to the resin, or addition of a thermoplastic oligomer or by silicone modification. However, with these methods, molded products tend to be cracked after immersion in the soldering hath, such that a reliable resin composition semiconductor encapsulation cannot be produced.
Meanwhile, phenolic resins are employed in the resin composition for semiconductor encapsulation as a curing agent for the epoxy resin. As such phenolic resins, novolak phenol resins or novolak cresol resins are employed. However, the phenolic resins lead to strong hygroscopic properties of semiconductor packages, as a result of which the above-mentioned cracking is inevitably produced on immersion of the semiconductor packages in the soldering bath.
For improving thermal resistance of the resin composition for semiconductor encapsulation, attempts have recently been made for improving the phenolic resin which is used as a curing agent for the epoxy resins. For example, in Japanese Laid-Open Patent Publication No. 3-66919 (1991), a phenol resin has been proposed which may be produced by reacting phenols with unsaturated polycyclic hydrocarbon compounds having two or more carbon-carbon double bonds, and a report has also been made that the aforementioned problems may be overcome by employing the phenol resin as a curing agent.
After the end of the reaction, acid catalysts or catalyst residues used for the reaction need to be removed completely because otherwise the resin composition for semiconductor encapsulation or the resin composition for laminated sheets containing the produced phenol resin tends to be lowered in their properties.
Although a method is known for washing the reaction liquid with water for complete removal of the acid catalyst or the catalyst residues during the preparation of the phenolic resin, this method is not useful in that the separation efficiency between the organic layer and the water layer after washing and the yield of the ultimate product are low and complex and labor-consuming operation and related equipment are required in disposal of the separated water layer and waste water.
Although there is known a method of washing the reaction liquid with an alkaline aqueous solution, this method is also not satisfactory in improving the separation efficiency between the oil layer and the water layer. Besides, the alkalis tend to produce colloidal hydroxide precipitates such that by-produced difficultly filterable precipitates need to be separated by an additional separating process.